> Using an extension tube on a zoom is
> very weird . . . . the needed extension
> varies based on focal length.
That's true, but you can use it to your advantage.
The change in angle-of-view and the change in
camera-to-subject distance tend to cancel each
other out at different zoom settings, so you can
get approximately the same magnification ratio
over a fairly wide range of camera-to-subject
distances. The zoom control acts like an "optical
focusing rail". :0)
Just put enough extension to get the ratio you want
and set the camera/tripod up somewhere close to
the right distance. Then you can use the zoom ring
to get the focus close, then fine-tune focus with the
focusing ring.
The optical quality of most zooms may leave
something to be desired, but it's nice not having to
shift the tripod back and forth and fiddle with the
amount of extension to get the magnification you
want.
-Don
